I feel right now there are core units for every compeditive idoneth list.
A) tide caster for cheap debuff and unbind (critical in new magic era and reducing powers of deathstar type units)
B) minimum 12+ eels, with a mix of ishlaen and morrsarr. Your hammer and anvil damage dealers
Appolexs, leviadons are too expensive or inefficent. You can take them (great models) they just wont make your points back.
After that its up to flavour. Thralls, reavers, aspect of the sea, king (and volturanos) if used properly can all have a place.
Reavers are good in a vaccum - they get less buffs, need less buffs and can be a bit throw away. I havent seen reavers do any serious heavy lifting or be Man of the match yet, but have seen them used well for plinking off wounds, seizing objectives, screening and surprising opponents with weight of attacks.
Aspect of the sea looks amazing and does a lot of cool thing BUT it takes practice to get 420 points worth. And some games he might struggle to get there but has definitly been man of the match for me unbinding, bebuffing, killing heroes etc. Aspect also has some really powerful combinations with artefacts.
The king/voluranos on paper are amazing but a bit odd. Those buffs are amazing, re-rolls, a round of bonus attacks, bravery. And they can hit really hard. But in this new era im finding spells and dangerous units that genrate low but reliable mortal wounds can make it hard to engage with king to make use of their combat abilities. Again you just need to think carefully and not discover you commited your king at the wrong time/got magic sniped a bit and died and suddenly lost all your precious buffs. This can mean wasting the kings combat till later turns which can feel like a let dowm.
Thralls are odd. Vicious in combat, not too expensive for a blob of 30 to be scary. But i feel you either go low 2 x 10 1 x20 or really invest in thralls to do some serious heavy lifting. They will die really quickly against some armies, but other armies do frighting amounts of damage at low model count. I guess if you go low thrall dont expect too much out of them (screening, summoning killers, munching through 1 wound units) but if you invest heavily in them some armies will hate you and some antihorde armies will wreck you. Unlike DoK our thralls are harder to just take 60 of though.
And as for more eels well...i still havent found what eels precisely do badly. Units of 3, 6, 9 all have their place, lots of buffs work well with them but they also have lots of special rules baked into their standard profile which are amazing. Fast, resilient, one shock cavlary, one rend resistant. Highish wound count and reasonable price means you can offset mortal wounds pretty well.
I must say the only problem with them (compedtively) if you dont know how to use them you real will struggle. Even with lists with only 12 and poinys heavily invested else where, they are critical for smashing enemy units and protecting your own. If you find your eels dying often and super quick you might need to check if your engageing at the right time and against the right enemies.
Or you can just take 9 morsarr, volutranos, and charge at high tide into allarielle the everqueen
1) 5 mortal wounds from eel shocks
2) large enough model and high move speed so all my eels could attack -
27 3+ rerolling 1's 3+ wounds rerolling wound -2 rends and 2 damage
3) didnt attack with mounts - she was dead.
Solid plan
